The most common and most likely answer to this crossword clue is the 11 letter word OUTSTANDING
I've seen this crossword clue in The Times
O
U
T
S
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A
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Buddy explains!

The straight part
We know that "exceptional" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case exceptional = outstanding.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OUTSTANDING mean?
OUTSTANDING verb- To resist effectually; withstand; sustain without yielding.
- To surpass in standing; stand or remain beyond; outstay.
- To project outward from the main body; stand out prominently; be prominent.
- To stand out to sea.
- To remain over; remain untouched, unimpaired, unsettled, uncollected, unpaid, or otherwise undetermined.
- Prominent or noticeable; standing out from others.
- Exceptionally good; distinguished from others by its superiority.
- Projecting outwards.
- Unresolved; not settled or finished.
- Owed as a debt.